1. Field of the Invention
The present invention relates to an optical information reproducing device, and more specifically, relates to an optical information reproducing device such as an optical pickup suitably used for a magneto-optical disk unit, in which the optical system is compact and integrated.
2. Description of the Related Art
An optical disk can store a huge amount of information with high density. In recent years, efforts for using such an optical disk have been in progress in various fields. Under such circumstances, it is important to provide a compact and lightweight optical information reproducing device, such as an optical pickup, which is used for reproducing information recorded on such an optical disk. It has been found that using a diffraction element for an optical system for the optical pickup is effective for this purpose, and some proposals have been presented to realize such an optical pickup using a diffraction element.
FIG. 9 schematically shows an optical system for a conventional optical pickup described in Y. Yoshida, et al., "Three beam CD optical pickup using a holographic optical element", Proc. Optical Data Storage Technologies, SPIE 1401 (1990), p. 58.
Referring to FIG. 9, divergent light emitted from a semiconductor laser 101 is introduced into a first diffraction element 102, where the incident light is split into a zero-order diffracted beam (main beam) and two first-order diffracted beams (sub-beams) for detecting a tracking error. Each of the split diffracted beams passes through a second diffraction element 103, and is converged on an information recording medium (hereinafter referred to as an "optical disk") 106 through a collimator lens 104 and an objective lens 105. The main beam forms a converged spot 108a on the optical disk 106, and the sub-beams form converged spots 108b and 108c on the optical disk 106. The light beams reflected from the optical disk 106 (hereinafter referred to as the "return beams") enter the optical system again through the objective lens 105, pass through the collimator lens 104, and are diffracted by the second diffraction element 103. The diffracted return beams are then converged on an optical detector 107. The first diffraction element 102 has a grating pattern of parallel straight lines with a predetermined grating pitch. The second diffraction element 103 has a grating pattern of curved lines determined so that the diffracted return beams can be properly converged on the optical detector 107.
Generally, a conventional optical information reproducing device has the following requirements: on an optical disk the light beam should be converged as a spot having a diameter of approximately 1 .mu.m; and such a spot should precisely follow the information track on the optical disk. To satisfy these requirements, a focusing error detection mechanism and a tracking error detection mechanism are indispensable for the optical information reproducing device.
First, the focusing error detection mechanism will be described.
As shown in FIG. 9, the second diffraction element 103 is substantially circular and has two semicircular regions 103a and 103b divided by a division line DL. The optical detector 107 has five detecting portions 107a, 107b, 107c, 107d, and 107e divided by four division lines A, B, C, and D.
Referring to FIG. 9, one part of the return main beam diffracted from the region 103a of the second diffraction element 103 forms a converged area 111a on the division line A of the optical detector 107, and the other part of the return main beam diffracted from the region 103b of the second diffraction element 103 forms a converged area 111a' on the detecting portion 107c of the optical detector 107.
Meanwhile, one of the return sub-beams reflected from the converged spot 108b on the optical disk 106 forms two converged areas 111b and 111b' on the detecting portion 107d of the optical detector 107. Likewise, the other return sub-beam reflected from the converged spot 108c on the optical disk 106 forms two converged areas 111c and 111c' on the detecting portion 107e of the optical detector 107.
In the above-described configuration, when the divergent light emitted from the semiconductor laser 101 is precisely focused on the optical disk 106 by means of the objective lens 105, the converged area 111a is formed right on the division line A as a focal point, as shown in FIG. 10B. As a result, the light amounts output from the detecting portions 107a and 107b are equal to each other.
On the other hand, in cases where the optical disk 106 is moved closer to the objective lens 105, the focal points of the diffracted return beams are formed behind the optical detector 107. As a result, as shown in FIG. 10C, the converged area 111a is formed on the detecting portion 107b in a semicircular shape. Reversely, in cases where the optical disk 106 is moved farther from the objective lens 105, the focal points of the diffracted return beams are formed in front of the optical detector 107. As a result, as shown in FIG. 10A, the converged area 111a is formed on the detecting portion 107a in a semicircular shape.
Thus, a focusing error signal FES is obtained by operating the following equation: EQU FES=S1-S2 (1)
wherein S1 and S2 are output signals from the detecting portions 107a and 107b of the optical detector 107. The operation is performed with a subtracter (not shown).
Based on the focusing error signal FES obtained as described above as a servo signal, the objective lens 105 is driven by an actuator (not shown) so as to move into a proper position where the converged spot 108a can be precisely on an information track 112 as a focal point.
Next, the tracking error detection mechanism for the above optical pickup will be described.
FIGS. 11A to 11C show the relative positions of the converged spots 108a, 108b, and 108c and the information track 112 on the optical disk 106. As shown in FIG. 11B, the converged spots 108b and 108c formed by the sub-beams are located apart from the converged spot 108a formed by the main beam by the same distance and are located in the opposite directions to each other along the information track 112. Further, the converged spots 108b and 108c are slightly shifted with regard to the information track 112 in the opposite directions to each other.
In the case where the information track 112 is shifted to the right with regard to the converged spot 108a due to some cause, as shown in FIG. 11A, the converged spot 108b is located substantially to the right on the information track 112. This results in that the intensity of the return beam reflected from the converged spot 108b decreases. On the contrary, the converged spot 108c is located farther from the information track 112, so that the intensity of the return beam reflected from the converged spot 108c increases.
On the contrary, in the case where the information track 112 is shifted to the left with regard to the converged spot 108a due to some cause, as shown in FIG. 11C, the converged spots 108b and 108c are located reversely to the former case. This results in that the intensity of the return beam reflected from the converged spot 108b increases, while that of the return beam reflected from the converged spot 108c decreases.
As described earlier, the sub-beams reflected from the converged spots 108b and 108c are converged on the detecting portions 107d and 107e of the optical detector 107, respectively. Accordingly, a tracking error signal TES is obtained by operating the following equation: EQU TES=S4-S5 (2)
wherein S4 and S5 are output signals from the detecting portions 107d and 107e. The operation is performed with a subtracter (not shown).
Based on the tracking error signal TES obtained as described above as a servo signal, the objective lens 105 is driven by an actuator (not shown) in order to move to a proper position where the converged spot 108a can be precisely on the information track 112 as a focal point.
FIG. 12 shows the conventional optical pickup shown in FIG. 9 in a more concrete configuration. Referring to FIG. 12, the first diffraction element 102 and the second diffraction element 103 are formed on the bottom surface and the top surface of a glass substrate 109, respectively, by etching. The semiconductor laser 101 and the optical detector 107 are disposed inside a hermetically sealed package 110.
FIG. 13 shows another conventional optical pickup with a diffraction element, which employs a "one-beam" method for tracking error detection. In FIG. 13, like components are denoted as like reference numerals as those in FIG. 12 for simplification. This conventional optical pickup is different from the optical pickup shown in FIGS. 9 and 12 in that a single diffraction element 113 is formed on the bottom surface of a glass substrate 114, and that an optical detector 115 is divided into four portions. The focusing error detection mechanism and the tracking error detection mechanism for this conventional optical pickup are mostly similar to those for the optical pickup shown in FIG. 9. The description of these mechanisms is therefore omitted. Refer to Y. Kurata, et al., "CD optical pickup using a computer generated holographic optical element", Proc. Optical Data Storage and Scanning Technologies, SPIE, 1139 (1989), p. 161, for details.
Such conventional optical pickups using a diffraction element and integrated as described above have an advantage of being less influenced by the environment because the light source and the optical detector are disposed in the same package. Also, they can be compact and lightweight, and can be fabricated at a reduced cost.
In recent years, with the spread of optical disks, a magneto-optical disk has attracted attention as a rewritable optical disk. FIG. 14 schematically shows an optical system for a conventional optical pickup directed for a megneto-optical disk by use of a diffraction element.
Referring to FIG. 14, divergent light emitted from a semiconductor laser 201 passes through a diffraction element 202 as a zero-order diffracted beam, and is converted into a parallel beam by a collimator lens 203. The parallel beam passes through a composite prism 204, a 45.degree. mirror 205, and an objective lens 206 so as to be converged on a magneto-optical disk 207.
The light beam reflected from the magneto-optical disk 207 (the return beam) enters the optical system again through the objective lens 206. Part of the return beam passes through the composite prism 204 and the collimator lens 203, and is diffracted by the diffraction element 202 so as to be converged on an optical detector 209 for detecting a focusing error and a tracking error. The diffraction element 202 has a pattern of curved lines determined so that the diffracted return beam can be properly converged on the optical detector 209, though not shown in FIG. 14. Since the focusing error detection mechanism and the tracking error detection mechanism for this optical pickup are similar to those for the optical pickup shown in FIG. 9, the description of these mechanisms is omitted. Refer to Sato, et al., "A pickup for a 3.5-inch magneto-optical disk", Sharp Technical Report, No. 50 (1991), pp. 20-24, for details.
The remaining part of the return beam is reflected twice in the composite prism 204 so as to reach a Wollaston prism 210, where it is split into two different polarized beams. The polarized beams are then converged on a two-division photo diode 212 through a converging lens 211. The difference of outputs from the two divisions of the photo diode 212 is calculated so as to reproduce an information signal.
As is known, when P-polarized light, for example, is radiated onto a position on a magneto-optical disk, the light is reflected from the position with the plane of polarization of the light rotated by .theta.k or -.theta.k determined corresponding to the direction of magnetization at the position irradiated with the light (refer to FIG. 15). The degree of this rotation is detected as an information signal. Generally, in order to detect the degree of the rotation, the return light is split into two polarization components having .+-.45.degree. directions, and the difference of the intensity of the two components is calculated. The crystal orientation of the Wollaston prism 210 is determined so that such polarized components can be obtained.
The optical pickups for an optical disk shown in FIGS. 9 and 13 which can be compact and light-weight are suitable for a reproduction-only type disk unit and a write-once type disk unit. However, these optical pickups are not provided with the function of detecting a magneto-optical signal, and thus are not applicable to a rewritable magneto-optical disk unit. Meanwhile, the optical pickup for a magneto-optical disk shown in FIG. 14 includes a prism as a component of the optical system for detecting a magneto-optical signal. This makes it difficult to drastically reduce the size and weight of the optical pickup, though the reduction is possible to some extent by using a diffraction element. Furthermore, the Wollaston prism used for polarization splitting is made of expensive crystal, thereby preventing a cost reduction of the optical pickup.